1. Field of the Invention
The present invention generally relates to a power level measuring device and a mobile station in a mobile communications system having a plurality of cells.
2. Description of the Related Art
In a mobile communications system including a plurality of cells, a mobile station generally detects the cell transmitting the pilot channel having the largest reception power, when initializing or during an intermittent receiving situation. When performing soft handover, such a mobile station detects the cell transmitting the pilot channel having the second largest reception power.
In a W-CDMA communications system, three stage cell searches are performed, using frame timing, scrambling code groups detected by P-SCH, S-SCH, and scrambling code utilized for spreading CPICH (Common Pilot Channel).
The mobile station performs the cell search, and specifies scrambling codes and despreading timings.
The cell search is required for carrying out RAKE synthesis of signals received via multipath. For each path, CPICH is used to specify despreading timings (path timings).
The path timings are likely to move forward and backward depending on the communications environment. Therefore, the path search should be performed regularly even after path timing detection.
Based on the path timings obtained through path searching, the mobile station reports power levels from ambient cells to the network when required.
FIG. 1 is a functional block diagram of a prior art power level measuring device that can be mounted on a communication apparatus such as a mobile station of a mobile communications system.
The power level measuring device comprises a path searching unit 110 and a demodulation unit 130. The path searching unit 110 comprises a matched filter 112, a power calculator 114, a power accumulator 116, a peak detector 118, and is also called a “searcher”.
The path searching unit 110 has a cell searching function (three stage cell searching function) mentioned above. Path searching is explained below.
The demodulating unit 130 comprises a sliding correlator 132, a power calculator/synchronous detector 134, a RAKE synthesizer 135 and a level converter 136. Plural sliding correlators 132 performing despreading are provided, and they are used in despreading received signals with respect to each multipath or each channel.
Timing at which each sliding correlator 132 despreads is in accordance with path timing reported by the path searching unit 110.
Path searching and level measuring processes performed in the path searching unit 110 are explained below.
The matched filter 112 calculates correlation values cpich_symbol [m] [i] between a received signal “x” and known codes “c” for each sampling time within a predetermined range by the following Equation 1.
                                          cpich_symbol            ⁡                          [              m              ]                                ⁡                      [            i            ]                          =                              ∑                          k              =              1                        nscode                    ⁢                                          ⁢                                    x              ⁡                              [                                  t                  +                                      (                                          m                      -                      320                                        )                                    +                                      (                                          i                      ×                      1024                                        )                                    +                                      4                    ×                    k                                                  ]                                      ×                          code              ⁡                              [                k                ]                                                                        Equation        ⁢                                  ⁢        1            
wherein “m” is a parameter defining a sampling time, and m=0, 1, . . . , 638, 639. The symbol “i” is a parameter defining a data symbol number, and i=0, 1, 2, . . . . Summation is done in the range of k=1, 2, . . . , nscode. The nscode is the number of chips included in one symbol of a CPICH signal, for example 256 chips. The symbol “t” means a path timing, which is determined by path searching. Within ±320 samples from this path timing being center, 640 correlation values as a total are calculated per one symbol. In this prior art example, one chip is represented by four samples (four times over sampling). The relationship between “t” and “m” is illustrated in FIG. 2A. Code[k] means a spreading code, and may be represented by a product of a channelization code CC[k] and a scrambling code SC[k].
The power calculator 114 calculates a sum of squares of in-phase and quadrature coefficients of the correlation values to obtain a power value (the received signal x is modulated with a quadrature modulation method). Such a power value is calculated for each of the 640 correlation values obtained per symbol.
The power accumulator 116 accumulates the power values obtained over 640 samples, at each sampling time. Accordingly there appear plural paths near to path timing “t”.
The peak detector 118 selects four path timings t0, t1, t2 and t3 corresponding to the four largest paths near to “t”, based on the accumulation results in the power accumulator 116 (cf. FIG. 2(B)). Using each of the path timings, the heading frame of the following received signal is maintained properly for each multipath, and power measuring is performed in the demodulation unit 130.
The sliding correlator 132 uses the four path timings t0, t1, t2 and t3 reported by the peak detector 118, and calculates a correlation value cpich_symbol [n][i] between the following received signal and known signals by the Equation 2.
                                          cpich_symbol            ⁡                          [              n              ]                                ⁡                      [            i            ]                          =                              ∑                          k              =              1                        nscode                    ⁢                                          ⁢                                    x              ⁡                              [                                                      t                    n                                    +                                      (                                          i                      ×                      1024                                        )                                    +                                      4                    ×                    k                                                  ]                                      ×                          code              ⁡                              [                k                ]                                                                        Equation        ⁢                                  ⁢        2                            wherein “n” is a parameter defining four path timings, n=0, 1, 2, 3. Other symbols are the same as the above. In this manner, four correlations that are despread received signals are output for each symbol.        
The power calculator/synchronous detector 134 averages the despread signals and outputs the averaged signals to the RAKE synthesizer 135. The power calculator/synchronous detector 134 can perform power calculation when measuring power level, and can perform synchronous detection when demodulating data. When demodulating data, data after RAKE synthesizing are output to a demodulating part (not shown), not to the level converter 136.
When a base station is under a transmission diversity condition, averaging is performed after pilot-canceling a received signal from one antenna, and RAKE synthesis is performed similarly.
The RAKE synthesizer 135 performs RAKE synthesis on each path (four paths), and outputs RAKE synthesized signals to the level converter 136.
The level converter 136, based on the RAKE synthesized signals, calculates a signal-to-noise power ratio Ec/No per chip. By multiplying this value with received signal strength RSSI, adequately converted power level RSCP is calculated.
The received signal strength can be determined by a radio unit (not shown) provided before the power measuring unit, based on a control signal of an automatic gain controller.
Thus calculated level measured value is transmitted to a network controlling device (RNC) via a radio base station, and utilized for handover control or other processing.
FIG. 3 is a timing chart with respect to level measuring and path searching.
As shown in the upper portion of FIG. 3, a radio signal received at a mobile station comprises a frame (10 ms) including a plurality (15) of time slots.
The level measuring is performed for plural cells (#0-#3) during one frame, and cells to be level-measured are switched at every 1.5 slots in the shown example.
As shown in the upper portion of FIG. 3, cell #0 is level-measured at the head of one frame. Next, cell #1, then cell #2, cell #3, and again cell #0. In this manner, despreading codes set in the sliding correlator 132 are switched so that each cell is level-measured twice during one frame.
The reason why 1.5 slots are allocated to the level-measuring time for each cell is to consider switching time for despreading codes. One slot is consumed for measuring power and switching despreading codes is done during 0.5 slots as overhead.
The middle portion of FIG. 3 shows path searching periods.
As shown in FIG. 3, a path search for one cell is performed during one frame, and cells to be path-searched are switched at every frame.
That is, despreading codes given to the matched filter 112 are switched at every frame.
In this example, the path search results for each cell are reflected (revised) immediately after measurement.
That is, as for cell #0, the path search results obtained during the first frame are applied to the second frame-the sixth frame. The level measurement during the period is carried out so that the sliding correlator 132 performs despreading according to the path timing detected in the first frame.
Next cell search for cell #0 is performed in the sixth frame. The level measurement on and after the seventh frame is performed based on path timings detected during the sixth frame.
The detected path timings can be given to a despreading processing unit for demodulating data.
The lower portion of FIG. 3 shows level measuring time by width of bars. Only Cell #0 is shown, but level measurement is also done for cell #1, cell #2 and cell #3, of course.
Correlation shown by a solid line typically represents that the reception level varies due to fading. It can be seen that the level measuring period is dispersed and therefore the influence of fading is well suppressed by averaging by power calculator/synchronous detector 134.
Another prior art communications system is described, for example, in Japanese Laid-Open Publication No. 11-317694.
The mobile station uses both the correlator in the path searching unit and the correlator in the demodulation unit, and therefore consumption of power in the correlators becomes twice as much.
It is one object of the present invention to reduce power consumption required for power level measuring.
Especially when a sliding correlator is used for correlation detecting, a plurality of multiplying circuits are provided for correlation calculation resulting in large circuit size. In this case, it is highly desirable to reduce power consumption.
A mobile station goes into a waiting mode in order to reduce power consumption, except while it is used for talking. A network may give an instruction to a mobile station anytime and the mobile station has to measure power level even while it is in the waiting mode. Then the mobile station starts the demodulation unit 130 and consumes power for purposes other than talking. This is disadvantageous especially for small mobile phones having limited battery capacity.
The received signal used by the peak detector 118 for determining path timings t0, t1, t2 and t3 is different from the received signal used by the sliding correlator 132 for despreading for level measurement.
That is, in FIG. 3, the path detected by path searching for cell #0 during the first frame is applied to the following second and sixth slots. The received signal used for path searching and the received signal used for level measuring do not have the same position.
Since the path search is done within any one frame in the path revision period, it is difficult to deal with environmental variation in the path revision period.